In conveyor-screw drives, the drives for plasticizer-worm extruders and machine tools, it is common practice to provide a leadscrew or other screw-carrying spindle and to drive the latter with an electric motor. A member, e.g. a nut, is displaceable axially along the screw by the rotation thereof.
In order to prevent overloading of the system, it has been proposed (see German printed application -- Auslegeschift DT-AS 11 98 159 corresponding to U.S. Pat. No. 3,369,087) to provide a bearing assembly for the spindle shaft which comprises a bearing housing and a pair of thrust-radial bearings, i.e. so-called shoulder bearings, acting in opposite directions on abutments provided on the shaft. The bearing housing, rather than being fixed, is permitted to shift axially between a pair of fixed plates against a plurality of compression springs. The bearing housing acts upon a pair of switches flanking this housing and actuated, depending upon the direction of rotation of the spindle, by the axial displacement of the housing when a reaction force is applied thereto because further movement of the nut is blocked or severely restricted. The switches are connected to the motor to deenergize the latter and thus prevent overloading of the device. The blockage of the nut can result from the latter reaching the end of its travel in one or the other direction, or because of some failure or other impediment in the travel of the nut.
While this system has been found to be highly effective in practice, it has the disadvantage that the reaction force described also results in some reverse rotation of the screw that can cause the housing to recede from the actuated switch and thereby bring about reenergization of the motor. As a result, the system is not turned off securely and reliably and vibration of chattering can ensue. This is obviously a drawback of the prior system described above.